For firefox users, dotjs(https://addons.mozilla.org/en-us/firefox/addon/dotjs/) does roughly the same thing but with custom js support as well.
for custom css... I clearly remember that old firefox version could let me set custom css, no plugin needed...
[1] http://www-archive.mozilla.org/unix/customizing.html [2] http://code.google.com/p/chromium/issues/detail?id=2393 [3] http://windows.microsoft.com/en-US/windows7/Use-your-own-sty... [4] http://www.opera.com/docs/usercss/#user-mode